Sometimes, the right set of Vise-Grips locking pliers on a rounded bolt head will work but you gotta clamp it down really hard on the bolt head.
Or a slightly too small "disposable" socket --- I sometimes get a cheap socket that I know will split apart after getting pounded onto it. That reminds me, I've got some lock lug nuts I need to remove from my car "before it's too late". (Can't find the lock key.)